What does the black rectangle clipper when filming films and movies actually do? Why do they need one at all?

Question

I always wondered this

In: 133

Answer 1

Took a film tour at Disney Hollywood Studios once.

They explained that the clapper was to sync up the video and audio streams, which were recorded separately– the camera just does the looks and they had a detached microphone for sound.

It’s likely this is still how its done despite that there are cameras that have built-in sound. But they would line up the “clap” in the audio with the moment that the bar closed on the board on camera, so it was easy to get the feeds to come together properly.